For atmospheric flight by heavier-than-air vehicles, it is well known that airfoils can be used in various ways to either propel or control the flight of the vehicle. For example, propellers are airfoils; the wings of airplanes are airfoils; and the rotor-blades of helicopters are airfoils. Broadly defined, an “airfoil” is a part or a surface, such as a wing, a propeller blade or rudder, whose shape and orientation control the stability, direction, lift, thrust, or propulsion of an aerial vehicle. For the purposes of the present invention, an airfoil is to be generally considered as an aerodynamically shaped, elongated blade that defines a longitudinal axis which extends from the root of the blade to its tip. The blade also defines a chord line that extends from the leading edge of the blade to its trailing edge, and that is generally perpendicular to the blade axis. As is well known, various configurations of airfoils have been designed and constructed for different kinds of aerial vehicles. The more commonly known vehicles that incorporate airfoils include: airplanes, helicopters, auto-gyros, rockets, and tilt-wing aircraft.
It has long been an objective in the design of manned aerial vehicles, to provide a vertical take-off and landing (VTOL) aircraft that is capable of performing within an extended operational flight envelope. Insofar as speed is concerned, such an aerial vehicle would preferably have a flight envelope that extends from hovering flight (i.e. zero velocity) to high-speed flight (e.g. greater than 250 knots). Further, it would be desirable for such a vehicle to have the capability of effectively, efficiently, and smoothly transitioning from one flight mode (e.g. hover, slow flight and high-speed flight) to another flight mode.
With certain limitations, helicopters and tilt wing aircraft are examples of aerial vehicles that have been designed with many of the above-mentioned objectives in mind. In this development and design process, however, other vehicle configurations have also been developed. As early as the 1930s, there was some experimentation with the so-called cycloidal propellers. Specifically, these propellers each incorporate several blades which move on respective cycloidal-type paths as they rotate about a common axis. Cycloidal propellers have the common characteristic that the respective longitudinal axis of each blade remains substantially parallel to a common axis of rotation as the propeller is rotated. In another aspect, however, cycloidal propellers can be rotated in either of two modes. One mode (curtate) is characterized by a blade movement wherein the chord line of the blade remains substantially tangential to the rotational path of the blade around the common axis. Another mode (prolate) is characterized by a blade movement wherein the chord line of the blade remains substantially parallel to the flight path of the vehicle as the blade is rotated around the common axis. In particular, a discussion of prolate flight is provided by U.S. Pat. No. 2,045,233 which issued in 1934 to Kirsten et al. for an invention entitled “Propeller for Aircraft.” In contrast with the cycloidal modes of blade movement, if the blade is stabilized, so as not to rotate around the common axis, a third mode (fixed wing) is established. It happens that each of the above-mentioned flight modes has its advantages.
In light of the above, it is an object of the present invention to provide a system for moving an aerial vehicle which can propel and control the vehicle at hover and slow flight (curtate flight mode), at intermediate flight speeds (prolate flight mode), and in high-speed cruise (fixed wing mode). Another object of the present invention is to provide a system for moving an aerial vehicle that can effectively, efficiently, and smoothly transition between the curtate, prolate and fixed wing modes of flight. Yet another object of the present invention is to provide a system for moving an aerial vehicle which is simple to operate, relatively easy to manufacture, and comparatively cost effective.